For oil country tubular goods, high-strengthening is demanded with deepening wells such as the oil-wells and the gas-wells (hereinafter, collectively simply referred to as an “oil-well”). Conventionally, oil country tubular goods of 80 ksi grade (yield strength is 80 to 95 ksi, that is, 551 to 654 MPa) or 95 ksi grade (yield strength is 95 to 110 ksi, that is, 654 to 758 MPa) have been widely used. However, in recent years, oil country tubular goods of 110 ksi grade (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa) is starting to be used.
Furthermore, most of deep wells, which are developed in recent years, contain hydrogen sulfide having corrosivity. Therefore, it is necessary for the oil country tubular goods to have not only high strength but also sulfide stress cracking resistance (hereinafter, referred to as SSC resistance).
Methods in which steel is purified or a steel structure is refined are known as an improvement plan for the SSC resistance of the conventional oil country tubular goods of 95 to 110 ksi grade. For example, Japanese Unexamined Patent Application, First Publication No. S62-253720 suggests a method of improving the SSC resistance by reducing impurity elements such as Mn, P, and the like. Japanese Unexamined Patent Application, First Publication No. S59-232220 suggests a method of improving the SSC resistance by carrying out a quenching treatment twice to refine crystal grains.
In response to the demand for the high-strengthening of the oil country tubular goods, in recent years, steel for oil country tubular goods of 125 ksi grade (yield strength is 862 to 965 MPa) has been suggested. However, the sulfide stress cracking (SSC) tends to occurs with an increase in the strength. Therefore, with regard to steel for oil country tubular goods of 125 ksi grade or higher, a further improvement in the SSC resistance is required as compared to the conventional steel for oil country tubular goods of 95 ksi grade or 110 ksi grade.
Japanese Unexamined Patent Application, First Publication No. H6-322478, Japanese Unexamined Patent Application, First Publication No. H8-311551, Japanese Unexamined Patent Application, First Publication No. H11-335731, Japanese Unexamined Patent Application, First Publication No. 2000-178682, Japanese
Unexamined Patent Application, First Publication No. 2000-256783, Japanese Unexamined Patent Application, First Publication No. 2000-297344, Japanese Unexamined Patent Application, First Publication No. 2000-119798, Japanese Unexamined Patent Application, First Publication No. 2005-350754, Japanese Unexamined Patent Application, First Publication No. 2006-265657, Japanese Unexamined Patent Application, First Publication No. 2000-313919, and PCT International Publication No. 2007/007678 suggest improvement plans for the SSC resistance of high-strength steel for oil country tubular goods.
Japanese Unexamined Patent Application, First Publication No. H6-322478 suggests a method of improving the SSC resistance of steel materials of 125 ksi grade by refining the steel structure through an induction-heating heat treatment. Japanese Unexamined Patent Application, First Publication No. H8-311551 suggests a method of improving the SSC resistance of steel pipes of 110 ksi grade to 140 ksi grade in a case of raising hardenability by using a direct quenching method and raising a tempering temperature. Japanese Unexamined Patent Application, First Publication No. H11-335731 suggests a method of improving the SSC resistance of low-alloy steel of 110 ksi grade to 140 ksi grade through an adjustment to an optimal alloy component. Japanese Unexamined Patent Application, First Publication No. 2000-178682, Japanese Unexamined Patent Application, First Publication No. 2000-256783, and Japanese Unexamined Patent Application, First Publication No. 2000-297344 suggest a method of improving the SSC resistance of low-alloy steel for oil country tubular goods of 110 ksi grade to 140 ksi grade by controlling the morphology of carbides. Japanese Unexamined Patent Application, First Publication No. 2000-119798 suggests a method of retarding an SSC occurrence time of steel materials of 110 ksi grade to 125 ksi grade by allowing fine V carbides to sufficiently precipitate. Japanese Unexamined Patent Application, First Publication No. 2005-350754 suggests a method of improving the SSC resistance of oil country tubular goods of 125 ksi grade or higher by controlling dislocation density and a hydrogen diffusion coefficient to desired values. Japanese Unexamined Patent Application, First Publication No. 2006-265657 suggests a method of improving the SSC resistance of steel for oil country tubular goods of 125 ksi grade or higher by allowing a large amount of C to be contained, stopping water cooling at 400 to 600° C. during the water cooling, and carrying out an isothermal transformation heat treatment (austempering treatment) at 400 to 600° C. to form a bainite single-phase structure. Japanese Unexamined Patent Application, First Publication No. 2000-313919 and PCT International Publication No. 2007/007678 discloses a method of improving the SSC resistance of steel pipes by increasing Mo content as compared to conventional oil country tubular goods.